[ruby-core:43302] [ruby-trunk - Feature #6070][Rejected] The scope of for loop variables

Issue #6070 has been updated by Shugo Maeda.

Status changed from Open to Rejected

Koichi Sasada wrote:
> (2012/03/05 16:10), Shugo Maeda wrote:
>  > For loops are syntactically close to while loops, but for loops are
>  > semantically more close to blocks, so I think it's better to make for
>  > loops consistent with blocks.
>  
>  I like current behavior because of consistency with "while" statement :)

Matz has the same opinion, so I'll close this ticket.

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Feature #6070: The scope of for loop variables
https://bugs.ruby-lang.org/issues/6070#change-24603

Author: Shugo Maeda
Status: Rejected
Priority: Normal
Assignee: Yukihiro Matsumoto
Category: core
Target version: 2.0.0


Hi,

In Ruby, the scope of a for loop variable is not limited in the for expression,
which means that a for expression counts on side effects.

This sometimes causes unexpected behavior when closing a for loop variable using a closure.

  $ cat test.rb
  procs = []
  for lang in ["Ruby", "Scala", "Haskell"]
    procs << -> { p lang }
  end
  procs.each(&:call)
  $ ruby test.rb
  "Haskell"
  "Haskell"
  "Haskell"

Why not make a for loop variable local to the for expression like a block parameter?
In Ruby 1.8, a for expression is faster than a method call with a block, but it's not
true in Ruby 1.9, so there is no reason to give a for expression special treatment.

The compatibility might be a problem, but I believe that code depending on the current behavior is evil.
Ruby's for expression also allows a global variable and a method call as a for loop variable.  However,
I rarely see such code in real-world applications.

Furthermore, I've heard that the scope of a foreach loop variable is changed in C# 5.

  http://blogs.msdn.com/b/ericlippert/archive/2009/11/12/closing-over-the-loop-variable-considered-harmful.aspx?PageIndex=3

Why should Ruby be more conservative than C#?

I've attached a patch for POC.

There is at least one problem in this patch.  The problem is that it cannot handle
the following code in mkmf.rb.
# I suspected that this code was written by nobu, but it was written by Eric.

  for lib in libs = $libs.split
    ...
  end

It's because libs is considered local to the for expression in parsing phase, but the variable
can't be found in dyna vars in compiling phase.  I'm wondering how this code should behave and
how to implement it.



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